This invention relates to electrostatic printing methods and compositions and more particularly to an improved ink and liquid developer composition for converting a latent electrostatic image to a visible image, the fixed image having improved rub resistance.
Processes for forming latent electrostatic images, existing as electrostatic charge patterns upon a substrate, and for subsequently converting the latent electrostatic image into a visual pattern, are well known. Generally, such electrostatic techniques have been carried out by using toners which are dry powders. However, many techniques have been developed in which the toner particles are suspended in a liquid carrier, and in electrostatic printing wherein latent electrostatic images are formed on a photoconductive surface of a recording element by uniformly charging the surface thereof, as by a corona discharge device, followed by exposure to light in the desired image pattern, such images may also be developed by liquid developers.
Liquid developer compositions for developing electrostatic images generally comprise a dispersion of pigment or toner particles in a volatile, insulating liquid of high dielectric strength and high volume resistivity, generally in excess of 10.sup.9 ohm-cm. The dispersed particles may carry either a positive charge or a negative electrical charge, depending upon their chemical composition, the non-conductivity and the high dielectric strength of the volatile liquid in the liquid developing composition preserves the electrostatic image permitting the deposition of the dispersed toner particles to form a visible image. Generally these liquid developer compositions have been commercially available inks comprising water or oil bases. Other suitable insulating liquids have included aromatic hydrocarbons, such as, benzene, toluene, and xylene; aliphatic hydrocarbons, such as, hexane, cyclohexane, and heptane; freons and halogenated hydrocarbons; and silicone oils.
Liquid toners or developers are described in U.S. Pat. No. 2,907,674, U.S. Pat. No. 2,899,335, U.S. Pat. No. 2,890,911, U.S. Pat. No. 3,135,095, U.S. Pat. No. 3,155,546, and U.S. Pat. No. 3,535,244.
In one type of liquid development the suspended toner particles are electrostatically charged and develop the latent image by migration of the particles to the image surface under influence of the image charge. This is known as electrophoretic development and utilizes the developers having insulating liquids of relatively high volume resistivity.
In another type of electrostatic image development disclosed by Gundlach in U.S. Pat. No. 3,084,043 and U.S. Pat. No. 3,551,146, liquid developers having relatively low viscosity, low volatility, contrast in color in the usual case to the surface on which it will remain, and relatively high electrical conductivity (relatively low volume resistivity), are disclosed for converting the electrostatic latent image to a visible image. According to this method liquid developer from a reservoir is deposited on a gravure roller and fills the depression in the roller surface. Excess developer is removed from the lands between depressions, and as a receiving surface charged in image configuration passes against the gravure roller, the liquid developer is attracted from the depressions in image configuration by the charge. This method of development is referred to as polar liquid development.
Although many inks and liquid developing compositions are known, none of the inks or liquid developers are entirely satisfactory from the standpoint of producing a permanent visible image which is free of smear or blurring especially when rubbed, that is, they do not have rub resistance. This results from the inability of the toner particles to become permanently fixed on the surface of the substrate to which the liquid developer or ink is transferred in image configuration. Attempts have been made to overcome this problem by fixing the image by subsequent processing, for example, by heating to fuse the toner particles, but such additional processing steps are generally undesirable.
Further attempts have been made to formulate the liquid developing composition by using a major portion of a low boiling liquid which is a nonsolvent for the organic binder, and a minor portion of a higher boiling liquid in which the organic binder is soluble, whereby the lower boiling liquid evaporates off more rapidly than the higher boiling liquid which thus increases in concentration towards the end of the drying cycle to the level whereby it becomes effective to reduce the organic binder to a state for fixing the colored particles. However, evaporation can be a problem with these volatile liquid developers which are organic in nature. The evaporation of the solvents results in pollution of the surrounding air, a very undesirable characteristic in view of present day efforts to reduce or eliminate contaminants in the air. Furthermore, such characteristics contribute to the inability to control the consistency of the ink because evaporation of the carrier upon standing will change the viscosity and other characterisitics of the liquid developer.
Still other attempts have been made to produce liquid developing compositions with a resinous or polymeric component dissolved therein as a binder, or with a liquid system containing a solvent which partially dissolves or softens a resinous component contained in the suspension or which forms a part of the pigment particles for the purpose of bonding the particles onto the copy sheet. However, when such organic binder components are dissolved or partially dissolved in the carrier there is an agglomeration of the toner particles with resulting separation in the developing composition and non-uniformity in the deposition of the toner or coloring agent particles for development of the visible image. Furthermore, this type of liquid developer dries on applicator rolls if a volatile solvent is used, and thereby causes staining of the original copies, sticking of the rollers, and cobwebbing in operation of applicator rolls, and if non-volatile solvent is used, the image obtained invariably has poor rub resistance.